1. Field of the Invention
The present invention relates generally to an acoustic microscope system that can suitably be employed in measuring quantitatively the elastic characteristics of a testpiece using ultrasonic beams. More particularly, the invention relates to an improvement of an ultrasonic probe for use in the acoustic microscope system.
2. Related Art
An acoustic microscope system is an effective device in materials science for evaluating the characteristics such as the elastic characteristics of a particular material. For this purpose, the vertical distance (Z) between the material as a testpiece to be analyzed and an ultrasonic probe is varied while irradiating ultrasonic beams toward the testpiece to obtain the output voltage (V) of returning waves. As shown in FIG. 1, the output voltage (V) is a function of the distance Z and minimum values of the output voltage (V) of returning wave alternate with maximum values to provide a profile that is generally referred to as a "V(z) curve". The abscissa of this graph plots the distance Z (.mu.m) and the ordinate plots the signal level (dB). The interval between adjacent maximum values (peaks) or minimum values (valleys) on the V(z) curve is called an "interference period .DELTA.Z", which is a very important parameter for evaluating materials of the testpiece. Since the interference period .DELTA.Z depends upon the velocity of the Rayleigh wave or compressional wave returning from a testpiece while radiating ultrasonic beams toward the testpiece, it is first determined from the V(z) curve of that testpiece and the velocity of the Rayleigh wave or compressional wave is then determined from said .DELTA.Z.
The V(z) curve is a curve indicative of the periodical variation of the output voltage relative to the amount of defocus. The period .DELTA.Z of the V(z) curve has a relation to the velocity C.sub.R of the Rayleigh wave and expressed by the following equation (1): ##EQU1## where C.sub.0 represents the velocity of water and f represents a frequency of the incident ultrasonic wave.
On the basis of the foregoing equation, the elastic characteristics of a material can be expressed.
In order to obtain the V(z curve, an acoustic microscope apparatus employs an ultrasonic probe. There are two classes of ultrasonic probes so called as "point-focus-beam type probes" and "line-focus-beam type probes" as discussed below.
The ultrasonic probe is provided with an acoustic lens and a transducer mounted on an upper surface of the acoustic lens for irradiating ultrasonic beams toward a testpiece and receiving various types of waves reflecting from the testpiece. The transducer of the point-focus-beam probes consists generally of a circular-shaped piezoelectric element formed of ZnO, for example, and a pair of electrodes disposed an upper and lower surface of the piezoelectric element. The acoustic lens has at a lower surface thereof a spherical concave surface so that incident ultrasonic waves are focused at a point on an extending direction from the center of the spherical concave surface. Since all incident waves are focused at a single point, probes of this type are called as the point-focus type.
Probes of the point-focus type have two major advantages, that is, first, they can produce a C-scan image; second, they are capable of measuring the acoustic velocity of a surface wave in terms of the average for all directions in the points on the surface of the testpiece material to be analyzed.
FIG. 2 is a schematic view showing the conventional acoustic microscope system. A testpiece 6 placed on a table 13 is immersed in a couplant 5 such as water or the like, which is brought into contact with an ultrasonic probe 3. An ultrasonic excitation source 1 generates a high-frequency burst signal which is supplied to the probe 3 via a directional coupler 2. The probe 3 converts the input electric burst signal into ultrasonic waves which is focused by an acoustic lens of the probe 3 and radiated towards the testpiece 6 through the couplant 5.
The ultrasonic signal that has been reflected and scattered by the surface of the testpiece 6 is received by the same acoustic lens and reconverted into an electric signal, which is supplied to a receiver 4 via the directional coupler 2.
The electric signal supplied to the receiver 4 is amplified and thereafter sent to a peak detector 7 which detects peaks of the received signal. The detected peak value is read into a central processing unit (CPU) 9 via an A/D converter 8 and used as data for constructing a V(z) curve. Using this data, a display unit 10 indicates an appropriate image of the V(z) curve.
The ultrasonic excitation source 1 may be an impulse generator or a tone burst generator.
The system shown in FIG. 2 also includes a Z-axis stage 12 updating the vertical distance Z between the ultrasonic probe 3 and the testpiece 6 for each sampling position. The output of the probe 3 obtained at each sampling point can be expressed as a function of distance Z. The resulting expression is referred to as the V(z) curve. The Z-axis stage 12 is controlled to move up and down by means of a controller 11 which is controlled by commands supplied from the CPU 9.
An example of the condition of the reflected ultrasonic signal as detected by the probe 3 as well as waveforms thereof are described below with reference to FIGS. 3A to 3E.
FIG. 3A is an enlarged sectional diagram of the probe 3 showing the radiation and reflection of ultrasonic waves as they relate to the probe 3. The probe 3 consists of an acoustic lens 3a and a circular-shaped ultrasonic transducer 3b. The ultrasonic transducer 3b is composed of a piezoelectric device and a pair of upper and lower electrodes disposed on both top and bottom surfaces, respectively, of the piezoelectric device. A lead wire 3c connects to both the upper and lower electrodes for connecting to an external circuit.
It is recognized that the returning ultrasonic wave includes two components, one being a vertical reflection wave W.sub.D which is reflected directly and vertically from the testpiece 6 and the other being the transversal Rayleigh wave (leaky surface acoustic wave) W.sub.R which irradiates from the surface of the testpiece 6 substantially within the incident Rayleigh angle .theta..sub.r. These two components interfere with each other and the result is received and detected as a reflection ultrasonic signal by the ultrasonic transducer 3b. The waveform of the vertical reflection wave W.sub.D is shown in FIG. 3B and the irradiated leaky surface acoustic wave (Rayleigh wave) W.sub.R in FIG. 3C. The wave of interference between in-phase components is shown in FIG. 3D and the wave of interference between 180.degree. out-of-phase components is shown in FIG. 3E. The term `interference` as used herein means the superposition of two wave components.
The peak values of the resulting interference wave are detected by the peak detector 7.
The V(z) curve for the peak values of the interference wave as a function of Z detected by the peak detector 7 is as illustrated in FIG. 1. Each of the maximum values (peaks) on the curve refers to the interference between in-phase wave components and each of the minimum values (valleys) refers to the interference between 180.degree. out-of-phase components. Of course, there are various types of V(z) curves other than that shown in FIG. 1.
The V(z) curve and the associated .DELTA.Z can be used to determine the acoustic velocities of the leaky surface acoustic wave, compressional wave, transversal wave, etc. with various materials of interest. Further, the elastic characteristics of the materials such as the differences in elastic modulus and density, Young's Modulus and, the differences in crystal size can be determined from the acoustic velocities of those waves.
Details of the conventional acoustic microscope system outlined above are given in journals such as "Kikai to Kogu (Machines and Tools)", November 1987, pp. 49-54, and "Zairyo (Materials)", December 1986, Vol. 35, No. 399, pp. 1-10.
The conventional line-focus-beam probes will now be described as follows with reference to FIGS. 4 and 5.
As shown in FIG. 4, an ultrasonic transducer 20 consisting of lower and upper electrodes 22, 24 and a piezoelectric element 23 is mounted on a flat top surface of an acoustic lens 21. The acoustic lens 21 has a cylindrical concave lens surface 21A on the side opposite to the side where the transducer 20 is mounted. A couplant (e.g. a water drop) 23 shown in FIG. 5 is provided between the concave lens surface 21A and a testpiece 26. The cylindrical concave lens surface 21A extends along a direction parallel to a longitudinal direction of the testpiece 26. FIG. 5 shows a cross section of the probe as taken in its longitudinal direction.
Focal point F shown in FIG. 5 is the point where an ultrasonic wave incident at the critical Rayleigh angle .theta..sub.r focuses in the water as the couplant 23, and a plurality of such focal points form a line directing along the longitudinal direction of the testpiece 26. Hence, the probe of this type is generally called as a line-focus-beam probe.
FIG. 5 refers to the case where an ultrasonic wave Wa irradiates from the concave lens surface 21A substantially within a critical Rayleigh angle .theta..sub.r. The radiated ultrasonic wave travels on through the surface of the testpiece 26 as a leaky surface acoustic wave (Rayleigh wave) LSAW, which irradiates and returns back to the transducer 20 from the surface as an irradiated leaky surface acoustic wave Wb. An ultrasonic wave Wc that travels straight down through the acoustic lens 21 returns vertically to the transducer 20 as a vertical reflection wave Wd.
By measuring the irradiated and reflected waves Wb and Wd returning from the surface of the testpiece 26 with the vertical distance Z to the testpiece 26 being varied, a V(z) curve is constructed. The V(z) curve can be used to determine the sound velocity and other parameters to evaluate the various characteristics of the material of the testpiece 26.
The technique of the acoustic microscope employing the line-focus-beam type ultrasonic probe as described above is disclosed by Kushibiki et al. in "Evaluation of Substrates for Elastic Wave Devices" on pages 21-28 of the collected papers read at the 25th Symposium of Communications Research Institute of Tohoku University entitled "Ultrasonic Electronics--New Piezoelectric Applications", February 1989.
In order to determine the leaky elastic surface (Rayleigh) wave LSAW with the conventional system shown in FIGS. 1 to 5, the input ultrasonic wave must be radiated into the couplant substantially within the critical Rayleigh angle. In practice, however, the radiated wave contains partially the compressional wave component, that is, a leaky surface skimming compressional wave, which is difficult to detect as a separate entity after propagation through the surface of the testpiece because the signal level of the transversal leaky surface acoustic wave is much greater than that of the compressional wave component thereof. Thus, since the detected compressional wave is low in accuracy, it has been difficult to measure accurately the elastic characteristics of a material to be analyzed finally.